Generally, as shown in FIG. 5, in a laser welding of stacked workpieces, for example, a laser head 3 is disposed above a surface of one workpiece 1 among two stacked plate-shaped workpieces 1 and 2, and while irradiating a substantially circular laser 4 from the laser head 3 toward the surface of the workpiece 1, the laser head 3 is moved linearly with respect to each of the workpieces 1 and 2, whereby each of the workpieces 1 and 2 is welded. Reference numeral 4 denotes a path of the laser.
In addition, a laser irradiation diameter A on the surface of the workpiece 1 is substantially identical at a weld starting-edge portion, halfway from the weld starting-edge portion to a weld ending-edge portion, and at the weld ending-edge portion. In order to improve an efficiency of laser energy, the smallest focusing beam diameter (focused state) is set so that an energy intensity of the laser 4 is maximized, and a penetrating capacity by the laser 4 is maximized.
However, as shown in FIG. 6(b), in the conventional laser welding method described above, inconveniences occur such as a recess 5 being formed in the surface of the workpiece 1 and a through hole being formed that passes through the workpiece 1 at the weld ending-edge portion after the welding has been completed. The bonding strength and the sealing characteristics of each of the workpieces 1 and 2 are reduced, and it is not possible to guarantee the welding quality of the workpieces 1 and 2. Note that the portion that is shown by reference numeral 6 denotes a portion that has solidified after having been melted by a laser.
The reason that the recess 5 or the through hole is formed in the surface of the workpiece 1 at the weld ending-edge portion of the workpieces 1 and 2 is that, in such welding using the laser irradiation, a material that has melted simultaneously with the formation of a keyhole 20 due to the laser irradiation forms a weld while being driven into the keyhole 20 toward the back side thereof with respect to the direction of the forward movement, and then the melted material that has been driven into the keyhole 20 cools and gradually solidifies.
However, at the weld ending-edge portion, the recess 5 or the through hole may be formed in the center portion when the material is not sufficient to completely fill the inside of the keyhole 20 because sufficient material has not been supplied to the inside of the keyhole 20 from the forward direction, and the melted material becomes cooler than the surroundings and contracts while solidifying.
Note that Patent Document 1 discloses a stacked laser welding method for zinc plated copper plates in which the beam shape at the irradiation position is an elliptical shape that has a major axis that is parallel to the welding line and a minor axis that is perpendicular thereto.    Patent Document 1: Japanese Patent Application Publication No. JP-A-2002-219590